The long-term goal of this project is to define the mechanisms that underlie pathogenesis of Langerhans cell histiocytosis (LCH). Specifically, I will investigate the role of genomic alterations and cell(s)-of-origin so that future therapies can be precisely tailored to molecular disease targets in each patient. Historically, the cellular and molecular causes of LCH have remained enigmatic, with diverse clinical presentations ranging from simple, self-resolving skin lesions to disseminated multi-organ involvement that is potentially fatal. Thus, treatment for the disease has remained empiric, based on clinical risk assessment?from resection of single skin lesions to systemic chemotherapy. More than half of patients remain unresponsive to front-line vinblastine/prednisone chemotherapy and often acquire permanent disease morbidities, such as diabetes insipidus. The first breakthrough in molecular disease pathogenesis, and a potential molecular target for therapy, (BRAF-V600E) was reported in 2010. Subsequent studies have provided more genomic, cellular and molecular evidence to support the hypothesis that LCH pathogenesis is the result of altered MAPK pathway activation at various stages of differentiation along the myeloid dendritic cell lineage. In additional to BRAF- V600E, alternatie mutually exclusive mutations have been discovered in BRAF and MAP2K1, accounting for ~90% of all LCH cases. This research is intended to define the pathogenesis of LCH at the genomic, cellular and molecular levels in order to promote design of therapies that are precisely directed at molecular targets, in effort to improve patient clinical outcomes over current front-line chemotherapeutics. We hypothesize that varied genomic alterations in LCH lesions activate MAPK pathway signaling in myeloid lineage origin cells to drive disease pathogenesis, and targeted therapy can block this signaling in pre-clinical LCH models. In Specific Aim 1, we plan to utilize an integrated genomic and transcriptomic approach to identify the genomic alterations in each LCH lesion, as well as the relative frequencies of such alterations. We will use clinical information to determine any statistically significant associations with LCH genomic alterations. Additionally, we will determine the hematopoietic lineages in which non-BRAF-V600E genome alterations occur. In Specific Aim 2, we will evaluate the impact of LCH genomic alterations on MAPK pathway activation, their sensitivity to varied MAPK pathway inhibitors and the gene expression profiles associated with specific alterations.